1. Technical Field of the Invention
The present invention relates to payload clock recovery in an ATM network node desynchronization for constant bit rate (CBR) services and, more particularly, to an improvement to a synchronous residual time stamp (SRTS) clock recovery technique.
The present invention thus also relates to the ATM adaptation layer (AAL) of asynchronous transfer mode (ATM) communication networks and, more particularly, to that class of services transported over the ATM layer in which the services are connection oriented, have a constant bit rate (CBR) and have a time relation between source and destination (SRTS-AAL, type 1; may also apply to AAL 5, where there is a relationship between frequency output and source clock at encoder, e.g., video).
2. Discussion of Related Art
The concept of a residual time stamp (RTS) is described in the article, "Synchronous Techniques for Timing Recovery in BISDN", by Richard C. Lau and Paul E. Fleischer, in IEEE Transactions on Communications, Vol. 43, No. 2/3/4, February/March/April 1995, pp. 1810-1818.
A known algorithm for carrying out this concept mimics the bit stuffing method of mapping a digital signal 1 (DS1) or digital signal 3 (DS3) payload into a virtual tributary (VT) or synchronous transport signal level 1 (STS1) synchronous payload envelope (SPE) already known from the synchronous optical network (SONET) standard promulgated by the American National Standards Institute (ANSI) by means of standard T1.105.x-199y. The known algorithm is only defined for the optimistic assumption that loss of synchronization traceability at different parts of the public telephone network is rare. The SRTS algorithm is defined in ANSI T1.630-1993, in Bellcore TA-NWT-001113, by the International Telecommunications Unit (ITU) in ITU-T Recommendation I.363, and ATM Forum specifications wherein SRTS is to be used for AAL 1 mapped CBR payloads such as DS1, DS3, and fixed bit rate video. As suggested above, the already-defined SRTS algorithm defines a normal mode condition, traceability to a network primary reference source (PRS) with a long-term accuracy of 10.sup.-11 or better, as well as a degraded mode condition (loss of PRS synchronization traceability), which is stated to not be part of the SRTS standardized definition.
If one were to implement the SRTS algorithm alone in a desynchronizer for a CBR service, when synchronization traceability to a PRS is lost, either temporarily or long-term, constant desynchronizer buffer spills could occur. The rate of buffer spills is dependent on the desynchronizer buffer size and the network synchronization frequency error between the mapping and desynchronizer nodes.